In recent years, to improve information-processing capability in electronic devices, frequencies of electrical signals transmitted through circuits on circuit boards used in the electronic devices have been increased to high frequency level. Therefore, even when the frequencies of the electrical signals are increased, the wiring (circuit) boards are required to maintain electrical reliability and suppress a decrease in electrical signal transmission rate and loss of electrical signals in the circuit.
Insulating layers, such as a protective film for protecting the circuit board and the circuit and an interlayer insulating film for ensuring insulation between layers constituting a multilayer circuit board, are usually provided on the above-described circuit board. The insulating layers such as the protective film and the interlayer insulating film are provided on the circuit board and thus have been required to have adhesiveness to the circuit board in addition to insulation properties.
In particular, when a multilayer circuit board, such as a build-up circuit board, is produced by lamination, the layers are bonded together with the interlayer insulating layer provided therebetween and fixed. At the same time, the space between lines in the circuit wiring are filled with a material constituting the interlayer insulating layer, thus fixing the lines. Therefore, the interlayer insulating film is required to have satisfactory adhesiveness to a substrate and the like and flowability such that the gaps between the lines in the circuit wiring are filled with the interlayer insulating film. Consequently, the insulating layers, such as the protective film and the interlayer insulating film, are composed of adhesive materials having adhesiveness and resin flowability.
Furthermore, in order to improve information-processing capability in electronic devices by increasing frequencies of electrical signals, even when insulating layers are composed of adhesive materials, the circuit boards are required to maintain electrical reliability in the gigahertz (GHz) band and have no adverse effect on the electrical signal transmission.
Conventionally, for example, epoxy adhesive materials and thermoplastic polyimide adhesive materials have been used as adhesive materials for circuit boards. The epoxy adhesive materials have satisfactory processability, i.e., the epoxy adhesive materials can bond adherends under low-temperature and low-pressure conditions and can fill the space between lines in circuit wiring. In addition, the epoxy adhesive materials have satisfactory adhesiveness to adherends. The thermoplastic polyimide adhesive materials have dielectric constants and dielectric loss tangents lower than those of epoxy resins and thus can be used in higher-frequency applications. Furthermore, the thermoplastic polyimide adhesive materials have, for example, satisfactory insulation (e.g., low volume resistivity) and excellent heat resistance (e.g., low thermal expansion and high thermal decomposition temperature).
Patent Document 1 discloses an adhesive being in the form of a film and composed of a mixture of a polyimide resin having a glass transition temperature within a predetermined range, an epoxy compound, and a compound having an active hydrogen group that can react with the epoxy compound, as a material containing the epoxy resin and polyimide resin described above, and this adhesive allowing bonding of adherends at a low temperature in a short time and ensuring reliability of heat resistance at a high temperature.
Patent Document 2 discloses a sealing epoxy resin composition containing an epoxy resin, a phenolic resin curing agent, a specific imide oligomer, and an inorganic filler.
Patent Document 3 discloses a hybrid adhesive composition containing an imide oligomer containing a specific repeating unit structure, an epoxy resin, and an epoxy curing agent.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 8-27430 (publication date: Jan. 30, 1996 (Heisei 8))    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 08-41172    [Patent Document 3] PCT Japanese Translation Patent Publication No. 2004-502859